Adding @abstractmethod __init__ to ABC breaks staticmethods

[euresti]

I was trying to create an ABC that also worked as a factory.

from abc import (
    ABCMeta,
    abstractmethod,
)

class WidgetWithInit(object):
    __metaclass__ = ABCMeta

    @abstractmethod
    def __init__(self, foo, bar):
        # type: (int, int) -> None
        pass

    @staticmethod
    def add(foo, bar):
        # type: (int, int) -> int
        return foo + bar

    @classmethod
    def add_class(cls, foo, bar):
        # type: (int, int) -> int
        return foo + bar

WidgetWithInit.add(1, '2')
WidgetWithInit.add_class(1, '2')

class WidgetNoInit(object):
    __metaclass__ = ABCMeta

    @staticmethod
    def add(foo, bar):
        # type: (int, int) -> int
        return foo + bar

    @classmethod
    def add_class(cls, foo, bar):
        # type: (int, int) -> int
        return foo + bar

WidgetNoInit.add(1, '2')
WidgetNoInit.add_class(1, '2')

MyPy will respond:

rt6.py:40: error: Argument 2 to "add" of "WidgetNoInit" has incompatible type "str"; expected "int"
rt6.py:41: error: Argument 2 to "add_class" of "WidgetNoInit" has incompatible type "str"; expected "int"

So adding the def __init__ seems to make the staticmethod and classmethod have a type of Any.

Fortunately I can work around it by not defining init in the ABC, and having my def create enforce the signature.

[rwbarton]

In fact mypy thinks the expression WidgetNoInit already has type Any with the definition

class WidgetWithInit(object):
    @abstractmethod
    def __init__(self, foo, bar):
        # type: (int, int) -> None
        pass

[rwbarton]

In fact __init__ having any decorator will result in the class being treated as Any, since type_object_type starts like this:

    init_method = info.get_method('__init__')
    if not init_method:
        # Must be an invalid class definition.
        return AnyType()
    else: ...

and get_method doesn't return decorated things.

[rwbarton]

Here's a possible test case

[case testAbstractInit]
from abc import abstractmethod, ABCMeta
class A(metaclass=ABCMeta):
    @abstractmethod
    def __init__(self, a: int) -> None:
        pass
class B(A):
    pass
class C(B):
    def __init__(self, a: int) -> None:
        self.c = a
a = A(1) # E: Cannot instantiate abstract class 'A' with abstract attribute '__init__'
A.c # E: "A" has no attribute "c"
b = B(2) # E: Cannot instantiate abstract class 'B' with abstract attribute '__init__'
B.c # E: "B" has no attribute "c"
c = C(3)
c.c
C.c

But I gave up on trying to fix this because the logic around recognizing decorators is too hairy.

[elazarg]

I think this bug is deep and important. I've encountered something similar in #3227. The logic that handles decorators seems to be flawed in a way which is difficult to fix.

[gvanrossum]

True, decorators are one of mypy's weak spots. We're slowly chipping away at it (e.g. #3918) but it's not easy because of special-casing in various places (both in Python and in mypy).